Thank you, there it is, the sound of the other shoe dropping ...
Microsoft Support said that ALL word documents attached to emails ALL open in Read Only mode.
That is the missing piece.
That is correct. A file attached to an email cannot be opened for editing. PERIOD.
I'm sorry, there is no "switch" to flip. The way that Outlook was originally designed does not allow for it. That is the way things are. You have to understand this and learn to work with the way things are.
You have to "detach" or "download" the file from the email to local storage, either your HD (or OneDrive) before you can open it for editing.